1. Field of the Invention
The present disclosure generally relates to image processing and, more particularly, to an image processing apparatus, storage medium, and method performed for outputting image data expressing halftone dots using an image forming apparatus.
2. Description of the Related Art
An offset printing method is a representative example of a printing technique which requires a plate-making process. Generally, in offset printing, proofing is performed to confirm a pattern, color tone, and text before performing the plate-making process to reduce plate-making cost. An image forming apparatus employing an inkjet method is often used as a prepress apparatus for performing proofing. More specifically, such an image forming apparatus forms an image by recording dots and thus does not require performing the plate-making process. However, image forming processes, color materials, and characteristics of the offset printing and the image forming apparatus employing the dot recording method are different. As a result, if the same image data is printed using offset printing and the image forming apparatus employing the dot recording method, the color tone and a dot layout become different between the printed images.
To solve such a problem, there is a technique for the image forming apparatus employing the dot recording method to faithfully reproduce the same image output by offset printing. More specifically, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2004-511184 discusses a method for performing multivalue conversion by applying a smoothing filter on a halftone dot image to be used in offset printing. By applying the smoothing filter, multivalue color tone information, which is expressed approximately as the halftone dot image to be printed by offset-printing, is obtained, and image processing considering usage of the image forming apparatus employing the dot recording method is performed.
However, when proofing is performed on the halftone dot image, reproduction of a halftone dot shape is necessary in addition to reproduction of the color tone. In particular, when offset printing is performed by overlapping the halftone dot images corresponding to a plurality of colors, it is desirable to reproduce moiré, such as a rosette pattern generated by overlapping the halftone dot images of each color, in the proofing. According to the method discussed in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2004-511184, the smoothing filter performs multivalue conversion for reproducing the color tone, so that the halftone dot shape becomes blurred. Reproducibility of the halftone dot shape thus becomes low.